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Towards a finite ensemble of ultracold fermions Fermi sea few fermions Timo Ottenstein Max-Planck-Institute for Nuclear Physics Heidelberg 19th International IUPAP Conference on few-body problems in physics Motivation • few fermion systems in nature – quarks in hadrons – nucleons in nuclei – electrons in atoms • challenging problems – pairing – energy spectrum of the system Ultracold atoms offer a simple and easy accessible model system in a tabletop experiment! Timo Ottenstein – 19th International IUPAP Conference on few-body problems 2 / 13 Our System scattering length (1000 a0) Ultracold 6Li atoms in the two lowest magnetic substates in an optical dipole trap Evaporative cooling ~100 nK 4 2 0 -2 -4 0 500 1000 1500 magnetic field (G) Interaction described by one single parameter the s-wave scattering length a! a can be tuned by means of broad Feshbach resonance! for a À range of interaction potential, properties of the system are universal! Timo Ottenstein – 19th International IUPAP Conference on few-body problems 3 / 13 Experimental Challenges 1. Preparation of a deeply degenerate Fermi gas with defined atom number Precise control over quantum states in the trap On demand with high fidelity! 1 2 N 2. Detection and quantitative measurements a) Counting single atoms b) Spatially resolved single atom detection Timo Ottenstein – 19th International IUPAP Conference on few-body problems 4 / 13 Our Approach state of the we aim for:art: obtain highFermi occupation degenerate gas (N~105) 1 inprobability a shallow close opticaltodipole trap, T ≈ 0.05 TF kBT P optical beam trap M. Bartenstein, et al., PRL 92, 12 (2004) w02 B. Demarco, et al., Science 285, 1703 (1999) micrometer size trap • thermal equilibrium: T/TF decreases by a factor of ~6! • switch off shallow trap Timo Ottenstein – 19th International IUPAP Conference on few-body problems 5 / 13 Control of the atom number “tilt the trap“ B (x) We aim for a control of the atom number on the single particle level. Timo Ottenstein – 19th International IUPAP Conference on few-body problems 6 / 13 The microtrap use high NA aspheric lens for microtrap focus (~3m) of a red detuned beam high field seeking atoms trap frequencies: r ~ 2 × 3.8 kHz z ~ 2 × 160 Hz r 4U 0 mw02 Timo Ottenstein – 19th International IUPAP Conference on few-body problems (P = 1 mW) 7 / 13 Atoms in the microtrap! N = 150.000 T = 200 nK T/TF = 0.27 N = 5.000 T = 200 nK deeply degenerate Timo Ottenstein – 19th International IUPAP Conference on few-body problems 8 / 13 Current status Apply magnetic field gradient after transfer of the atoms into the microtrap. Observed atomnumber statistics for highest value of magnetic field gradient. N = 120 +/- 11 Observed atom number fluctuations also caused by imaging technique! Go for single atom detection using fluorescence imaging. Timo Ottenstein – 19th International IUPAP Conference on few-body problems 9 / 13 Fluorescence imaging Proof of principle experiment: Measure fluorescence signal of single atoms in a weak Magneto-optical trap. CCD Fluorescence signal [a.u.] 3.0 3 atoms 2.5 2.0 2 atoms 1.5 1 atom 1.0 We are able to detect single atoms as discrete steps in the fluorescence signal on the CCD camera. 0.5 no atoms 0.0 0 50 100 150 200 250 300 Time [a.u.] Timo Ottenstein – 19th International IUPAP Conference on few-body problems 10 / 13 Conclusion - Ultracold atoms provide a clean and easy accessible model system for finite fermionic systems in nature Current status: - Implementation of a microtrap in the experimental setup - Control of the atom number in the regime of ~100 atoms - Fluorescence detection of single atoms in a weak MOT Next steps: - Design of a new lens system for a tighter focus and higher imaging resolution Timo Ottenstein – 19th International IUPAP Conference on few-body problems 11 / 13 Let’s do physics! • spill atoms from the trap with interaction switched on and compare with ideal gas case How do interactions change the energy of the system? For a , what is in a finite system? a0 • Probe single particle excitations in a finite Fermi system by radio frequency (RF) spectroscopy ? ideal Fermi gas |3> RF |2> |1> Timo Ottenstein – 19th International IUPAP Conference on few-body problems 12 / 13 Thank you! The ultracold quantum gases group @ MPIK Heidelberg Andre Wenz (currently @ UC Berkeley) Timo Ottenstein Friedhelm Serwane Gerhard Zuern Selim Jochim Thomas Lompe Timo Ottenstein – 19th International IUPAP Conference on few-body problems 13 / 13